1. Field of the Invention
The present invention relates to a hydraulic control device for an automatic transmission for a vehicle such as an automobile, and more particularly, to a hydraulic control device for such an automatic transmission having two parallelly arranged input clutches which are selectively engaged or disengaged in providing a plurality of speed stages.
2. Descriptions of the Prior Art
The hydraulically operated automatic transmission for a vehicle such as an automobile generally includes a hydraulically operated input clutch which is sometimes called a forward clutch and which, when engaged, connects the speed stage shifting gear mechanism of the transmission to a rotational power input member therefor which is generally a rotational output member of the fluid torque converter forming a power inlet portion of the automatic transmission. Such a forward clutch is engaged to provide all forward speed stages available by the transmission by a hydraulic pressure supplied thereto under the control of the manual shift valve when it is shifted to any one of forward drive ranges such as the so-called D range, S or 2 range, and L range. The manually operated shift valve generally never fails to provide a hydraulic pressure to the forward clutch when it is shifted to either one of the forward drive ranges. Therefore, it is generally improbable that the vehicle equipped with such a hydraulically operated automatic transmission is disabled to drive forward due to non engagement of the forward clutch.
In Japanese Patent Application No. 62-195471 filed on Aug. 5, 1987 and laid open to public on Feb. 13, 1989, two of the inventors of the present invention have proposed an automatic transmission for a vehicle such as an automobile in which a speed stage shifting mechanism includes two parallelly arranged input clutches and is adapted so that a first one of the two input clutches is engaged when it provides a first speed stage, a second speed stage and a third speed stage, whereas a second one of the two input clutches is engaged when it provides the third speed stage and a fourth speed stage. Therefore, with regard to the two input clutches there exist three conditions for forward driving of the vehicle that the first clutch only is engaged, the first and the second clutch are both engaged, and the second clutch only is engaged. In other words, the switching over of the two clutches is not an alternative selection. In order to operate such a transmission mechanism automatically, provided that the clutches are of a hydraulically operable type, normally at least two automatically operable hydraulic pressure switching over valves would be required. When two hydraulic pressure switching over valves are employed for the control of such two clutches, there is a probability that according to a sticking of either or both of the valves at one switching over position thereof none of the two clutches is supplied with hydraulic pressure. If it happens, no forward driving of the vehicle is available.